1. Field of the Invention
The present invention relates to a semiconductor device which is a junction-barrier Schottky diode having a trench structure, which semiconductor device is suitable as a Z diode (Zener diode) for use in the electrical system of a motor vehicle.
2. Description of Related Art
In modern motor vehicles, more and more functions are being implemented using electrical components. This results in an ever-increasing demand for electric power. To meet this demand, the efficiency of the generator system in the motor vehicle must be increased. So far, PN diodes have been usually used as Z diodes in the generator system of the motor vehicle. Advantages of PN diodes include low reverse current and great sturdiness. However, the main disadvantage is a comparatively high forward voltage UF. At room temperature, current begins to flow only at a forward voltage UF of approximately 0.7 V. Under normal operating conditions at which the current density is approximately 500 A/cm2, forward voltage UF increases to more than 1 V. This results in a decline in efficiency.
On the basis of theoretical considerations, the Schottky diode might be considered as an alternative. A Schottky diode has a much lower forward voltage than a PN diode. For example, the forward voltage of a Schottky diode is approx. 0.5 V to 0.6 V at a current density of approximately 500 A/cm2. Furthermore, a Schottky diode as a majority carrier component offers advantages in rapid switching operation. Inasmuch as is known, however, Schottky diodes have not yet been used in the generator system of a motor vehicle. This may be attributed to some significant disadvantages of a Schottky diode which make such an application appear to be less relevant. First, a Schottky diode has a higher reverse current than a PN diode. This reverse current also has a strong dependence on the reverse voltage. Finally, a Schottky diode has inferior sturdiness, in particular at high temperatures. These disadvantages have so far prevented the use of Schottky diodes in automotive applications.
Measures for improving the properties of Schottky diodes are already known from T. Sakai et al., “Experimental investigation of dependence of electrical characteristics on device parameters in Trench MOS Barrier Schottky Diodes,” Proceedings of 1998 International Symposium on Power Semiconductors & ICs, Kyoto, pp. 293-296, S. Kunori et al., “Low leakage current Schottky barrier diode,” Proceedings of 1992 International Symposium on Power Semiconductors & ICs, Tokyo, pp. 80-85, as well as DE 19 749 195 A1, resulting in the so-called JBS (JBS=junction-barrier Schottky diode) or the so-called TMBS (TMBS=trench-MOS-barrier Schottky diode). In the case of a JBS, the Schottky effect which is responsible for a high reverse current may be at least partially screened via suitable dimensioning of certain structural parameters and the reverse current thereby reduced. More extensive screening, e.g., by an even deeper p diffusion, however, is not feasible because at the same time the diffusion region would also be extended further in the lateral direction. However, the area available for current flow in the forward direction would then be further reduced in a deleterious manner. The advantage of a TMBS is the possible reduction in reverse current. Reverse current flows mainly along the surface of a trench formed in the diode structure through a quasi-inversion layer of the MOS structure of the diode. Consequently, the MOS structure may be degraded from an n-epi layer to an oxide layer by injection of “hot” charge carriers and may even be destroyed under particularly adverse conditions. Since a certain amount of time is needed for formation of the inversion channel, the space charge zone may briefly propagate further at the beginning of rapid switching operations and consequently the electrical field strength may increase. This may result in brief unwanted breakdown operation of the diode. It is therefore not very advisable to use a TMBS that has been improved with regard to the reverse current as a Z diode and operate it in the breakdown range.